Firstly, who gives a toss the ramifications of what Holden called their wheels, that Ron's referring to.....If it's an issue, let the owner of the trademark take it up with Holden..
2ndly...20" wheels will have a slightly harsher ride, due to the lack of sidewall give, that a 20" tyre will provide.
A 20" tyre will be slightly more expensive, because the demand isn't as great for these, as a 18" tyre has....From memory, 19" tyres are about the same price as the 20", due to demand.
I understand the frustration of having staggered rims fitted and how little you can do.....My suggestion would be, try and find a 18" decent rim that you like, that can be fitted front and rear, so you are able to rotate them and provide a comfortable ride....Maybe do some trade with the staggered rims you have, or just buy outright....
As for what was fitted from the factory....Maybe in 30 years, someone will want the car "factory original", when purchasing, but honestly these days, not having the original wheels on the car when selling/purchasing, only appeals to the minority of car buyers....